Project Summary. Microglia are the brain?s immune cells and have long been appreciated for their critical roles during brain injury and disease. Recent studies, however, have demonstrated that microglia have important roles in the absence of pathology and serve to maintain brain homeostasis, support developing neurons and aid in the remodeling of neural circuitry during development and plasticity. This makes microglia a therapeutic target for diseases in which neural circuits either develop or remodel inappropriately. Despite their importance in brain health and disease, very little is known about how microglia self-maintain after they colonize the brain during development. This knowledge is fundamental to understanding how microglia maintain their functions throughout the lifespan. While microglia are long-lived cells, depletion methods have demonstrated that the adult microglial population can be rapidly reconstituted by the proliferation of resident cells. Because these studies have been carried out in fixed tissue the dynamics and mechanisms of this process are unclear. I propose to determine how microglia are born and mature in vivo. I will also explore the role of the purinergic receptor, P2Y12 in this process. I hypothesize that microglia are born by the division of single microglia and depend on purinergic signaling to rapidly differentiate morphologically and functionally to perform their physiological roles in the brain. In order to test this hypothesis, I will address two related but independent aims. In Aim 1, I have started to test the hypothesis that microglia repopulate from rapid division of single surviving microglia under normal conditions and after experimental depletion. I have also shown that microglia are critical to experience- dependent plasticity in the adult visual cortex. In Aim 2, I will test the hypothesis that the physiological attributes of microglia such as process motility and microglia-synapse interactions return to baseline soon after microglia are born in the adult. I will also explore whether P2Y12 signaling contributes to the maturation of these attributes in newly-born adult microglia. Lastly, I will examine how rapidly newly-born microglia acquire their functions that support experience-dependent plasticity in the visual cortex. Together these two aims characterize how microglia proliferate and maintain homeostasis in the adult cortex. Thus, this proposal embodies a new set of techniques that allows the close monitoring of the self-renewal dynamics of microglia. The training I will obtain in graduate school while performing the first to aims, will prepare me for my postdoctoral studies in Aim 3, where I will carry out collaborative work to determine therapeutic targets for the treatment of neurodevelopmental disorders.